1. CD-ROM Appendix
The computer program listing appendix referenced, included and incorporated in the present application is included in a single CD-ROM appendix labeled “ACTIVE COLOR CONTROL FOR A PRINTING PRESS” which is submitted in duplicate. The CD-ROM appendix includes 72 files. The computer program is incorporated herein by reference.
2. Field of the Invention
The present invention relates a system for the accurate measurement and control of image color density on a printing press. More particularly, the invention provides a method and apparatus for controlling the amount of each of the inks used to print an image based on the color densities of each of the inks detected in printed images.
3. Description of the Related Art
Color perception of a printed image by the human eye is determined by the light reflected from the printed substrate. Changing the amount of ink applied to the substrate changes the amount of color on a printed substrate, and hence the quality of the perceived image.
Each of the individual single images is produced with a specific color ink. A multi-colored printed image is produced by combining a plurality of superimposed single color printed images onto a substrate. To create a multi-colored image, inks are applied at a predetermined pattern and thickness, or ink density. The ink patterns are generally not solid, but are composed of arrays of dots which appear as a solid colors when viewed by the human eye at a distance. The images produced by such arrays of colored dots are called halftones. The fractional coverage of the dots of a halftone ink pattern is referred to as the density of the ink pattern. For example, when ink dots are spaced so that half the area of an ink pattern is covered by ink and half is not, the dot density of the ink pattern is considered to be 50%.
The color quality of a multi-colored printed image is determined by the degree to which the colors of the image match the desired colors for the image, i.e. the colors of a reference image. Hence, the obtained quality of a multi-color image is determined by the density of each of the individual colored images of which the multi-colored image is composed. An inaccurate ink density setting for any of the colors may result in a multi-colored image of inferior color quality. An offset printing press includes an inking assembly for each color of ink used in the printing process. Each inking assembly includes an ink reservoir as well as a segmented doctor blade disposed along the outer surface of an ink fountain roller. The amount of ink supplied to the roller train of the press and ultimately to a substrate, such as paper, is adjusted by changing the spacing between the edge of the blade segments and the outer surface of the ink fountain roller. The position of each blade segment relative to the ink fountain roller is independently adjustable by movement of an ink control device such as an adjusting screw, or ink key, to thereby control the amount of ink fed to a corresponding longitudinal strip or ink key zone of the substrate. The ink control mechanism includes any device that controls the amount of ink fed to a corresponding longitudinal strip or zone of the substrate. The ink control keys each control the amount of ink supplied to a respective ink key zone on the substrate.
In the printing industry, color bars have been used for a long time to measure color density. A color bar comprises a series of patches of different colors in each ink key zone. To get the required density of the printed information on a substrate, the press operator measures the density of the color patches in the required ink key zone. The ink density of a color is determined by the settings of the ink supply for the ink of that color. A printing press operator adjusts amount of ink applied to the substrate to get the desired color. Opening a key increases amount of ink along its path and vice versa. If ink density of the patch is too low, the operator opens the ink key to increase amount of ink. If ink density of the patch is too high, the operator closes the ink key to decrease amount of ink. It is assumed that the change in color density of patches also make similar changes in the color density of the desired printed image. However, it is well known in the printing industry that this assumption is not always correct. To adjust for this discrepancy, press operator takes a color bar patch density only as a guide, and final color adjustments are made by visually inspecting the printed image and also by measuring density of critical areas in the print.
At the start of a printing run, the density settings for the various color inks must be set to achieve the appropriate ink density levels for the individual color images in order to produce multicolor images with the desired colors. Additionally, adjustments to the ink density settings may be required to compensate for deviations in the printing parameters of colors during a printing run. Such deviations may be caused by alignment changes between various rollers in the printing system, the paper stock, web tension, room temperature and humidity, among other factors. Adjustments may also be required to compensate for printing parameter deviations that occur from one printing run to another. In the past, such ink density adjustments have been performed by human operators based merely on conclusions drawn from the visual inspection of printed images. However, such manual control methods tended to be slow, relatively inaccurate, and labor intensive. The visual inspection techniques used in connection with ink key presetting and color control are inaccurate, expensive, and time-consuming. Further, since the required image colors are often halftones of ink combined with other ink colors, such techniques also require a high level of operator expertise.
Methods other than visual inspection of the printed image are also known for monitoring color quality once the press is running. Methods have been developed to control ink supplies based on objective measurements of the printed images. To conduct the task of color density measurement, offline density measurement instruments are available. Quality control of color printing processes can be achieved by measuring the optical density of a test target image. Optical density of various points of the test target image can be measured by using a densitometer or scanning densitometer either off-line or on-line of the web printing process. Typically, optical density measurements are performed by illuminating the test target image with a light source and measuring the intensity of the light reflected from the image.
A press operator may take a sample of a printed substrate with the color bars and place it in a test instrument. A typical instrument has a density scanning head traveling across the width of color bars. After scanning, the instrument displays density measurements on a computer screen. Upon examining the density values on display and also examining the printed sample, the operator makes necessary changes to the ink keys. This procedure is repeated until satisfactory print quality is achieved. Known methods for controlling printed colors have included the use of such a densitometer to measure the density of color bars printed in a margin outside the area of the printed image. In such prior art systems, one color bar is printed for each of the base colors, and the densitometer measures the ratio of light reflected from bare paper to light reflected from the color bars in order to determine appropriate ink density settings. A disadvantage of using a densitometer is that one must look to the separately printed color bars. If the colors of the color bars are true, then one must presume that the colors in the printed image are, therefore, also correct. However, if the colors in the color bar do not exactly represent the colors of the image, then the ink density settings based on the color bar measurements will be inaccurate. Another approach to controlling the printing of colors involves the use of a spectrophotometer which measures reflected light intensity as a function of wavelength. Spectrophotometer measurements represent the value of reflected light intensity in given spectrum segments of the light spectrum scanned. The spectrophotometer method determines the ink density for each color in an image. The spectrophotometer method also has the disadvantage of requiring one to presume that what is observed in a reference color bar correctly indicates what occurs in the printed image. A key disadvantage of these techniques is that they must be performed off-line. That is, a sample of the printed substrate must be measured either while the press is stopped or the sample taken away from the press. This involves considerable downtime and wasted printing during trial and error ink level correction.
To automate this task, online density measurement instruments are known. While the press is running, it is common for a press operator to continually monitor the printed output and to make appropriate ink key adjustments in order to achieve appropriate quality control of the color of the printed image. For example, if the color in a zone is too weak, the operator adjusts the corresponding ink key to allow more ink flow to that zone. If the color is too strong, the corresponding ink key is adjusted to decrease the ink flow. During operation of the printing press, further color adjustments may be necessary to compensate for changing press conditions, or to account for the personal preferences of the customer.
Online instruments comprise a scanning assembly mounted on the printing press. The test target image that is measured is often in the form of a color bar comprised of individual color patches. The color bar typically extends the width of the web (see FIG. 7). Typically, color bars are scanned on the printing press at the patches, which include solid patches and halftone patches for each of the primary ink colors, as well as solid overprints. The color bar is often printed in the trim area of the web and may be utilized for registration as well as color monitoring purposes. Each solid patch has a target density that the color control system attempts to maintain. The inking level is increased or decreased to reach this target density. Instruments that can measure density on the press and also automatically activate ink keys on the press to bring color density to a desired value are commonly known as Closed Loop Color Controls. Instruments vary in the way they scan color bars and calculate color patch density. Different scanning methods can be categorized into two groups. A first group uses a spectrophotometer mounted in the imaging assembly. A video camera and strobe are used to freeze the image of moving substrate and accurately locate color bars. The spectrophotometer then takes a reading of the color patch. For positioning color patches in Y direction, a cue mark and a photo sensor are used. For distinguishing color patches from print, a special shape of color patch is required for this instrument. A second group uses video cameras mounted in an imaging assembly. Typically, a color camera with a xenon strobe is used to freeze the motion of moving substrate and acquire an image. Most manufacturers use an analog 3 CCD camera, in which prisms are used to split red, green and blue channels. Analog signals from these three channels are fed to frame grabbing electronics to digitize and store image. Examples of these prior systems are disclosed in U.S. Pat. Nos. 5,543,922; 5,724,259; 5,967,049; 5,967,050, 5,992, 318; 6,058,201; and 6,318,260. It has been found according to the present invention that by use of a digital video camera, direct digital signals for each color channel are made available for processing without requiring analog to digital conversion which loses some degree of color resolution.
Prior patentees have also used flashing xenon strobes for illuminating the moving substrate for a short period of time. Xenon strobes work on the principle of high voltage discharge through a tube filled with xenon gas. However, it is well known that the light intensity from strobe to strobe is not consistent. This becomes a problem in color density measurement since variation in strobe intensity provides false readings. To overcome this problem, one patentee uses a light output measurement device in front of the strobe and provides correction in color density calculations. As an additional disadvantage, the xenon strobes work with a high voltage and drive electronics generate electrical noise and heat. These features make it more difficult to package a camera and xenon strobe in a single sealed imaging assembly. Another system therefore mounts the strobe away from camera and transmits light through a light pipe. In one embodiment of the present invention, this problem is solved by use of a high intensity white light emitting diodes instead of xenon strobes.
The invention not only concerns the measurement and determination of color density variations, but also a method for controlling the plurality of ink control mechanisms, or keys on a printing press for on-the-run color correction. The inventive system includes the process of measuring color values for selected images printed on the substrate to produce an acquired image array. Then, the acquired image is compared with a standard image array comprised of standard color values for the image areas. Ink adjustments are then calculated for each ink key zone and adjustment of the ink control mechanisms maintains the desired color density of the images.
Importantly, since the color bars do not always indicate the colors of the image to be printed, the invention allows a determination and adjustment of image density not only at color bars, but also at any portion of the entire printed image across the substrate. Once the desired key adjustments for a particular printing job are determined, the values of the key positions can also be stored in a computer memory for setup use at another time.